Optical absorption gas sensors include both a source of light and a light detector. Light from the source is directed through a gas sample and detected by the light detector. The concentration of an analyte gas in the gas sample can be determined from the absorption of light by the analyte gas. Typically, either the source will emit light predominantly within a wavelength range corresponding to absorption lines of the intended analyte, or the detector will be sensitive to light predominantly within a wavelength range corresponding to absorption lines of the intended analyte, either due to the inherent properties of the light sensitive transducer which is employed or due to the presence of a wavelength filter which selects only light within a wavelength range including absorption wavelengths of the target analyte. Within this specification and the appended claims, light refers to electromagnetic radiation irrespective of wavelength and includes, for example, electromagnetic radiation in the infra-red region of the spectrum as this is a region within which many analyte gases have strong absorption lines.
The invention relates in particular to optical absorption gas sensors in which the source of light is an LED (for example, an infra-red LED) and the light sensitive transducer is a photodiode.
Light emitting diodes (LEDs) and photodiodes are inexpensive and relatively energy efficient devices and so they are commonly employed as sources and detectors of light in optical absorption gas sensors, particularly devices which are intended to be small and low cost. For many applications, an LED with a peak emission wavelength in the infra-red region of the spectrum, and a photodiode which is sensitive to infrared radiation are suitable.
LEDs and photodiodes are sensitive to a number of environmental factors, including temperature. Therefore in order to provide accurate measurements of analyte gas concentration, it is necessary to regulate the temperature of the LED and photodiode, or compensate for the temperature of the LED and photodiode, or to adopt another strategy. For example, in WO 2007/091043 (Gas Sensing Solutions Ltd.) the LED and photodiode are mounted substantially in thermal equilibrium. This latter strategy is advantageous in that a single temperature measurement may be made, for example, a measurement of the temperature of the LED, and the temperature of the other component can be inferred to be the same. This reduces the degrees of freedom of the system, simplifying temperature compensation in use. However, it provides constraints on the design of optical gas sensors, as the LED and photodiode must be mounted close to each other in order to be in thermal equilibrium.
In theory, it may be possible to independently calibrate the variation in optical properties of the LED with temperature, and the variation in optical properties of the photodiode with temperature, by entirely isolating the devices or independently controlling their temperatures. However, in practical, cost-effective manufacturing procedures, it is preferable to be able to carry out calibration using only measurements of photodiode output current from an assembled sensor, responsive to light from the LED within the assembled sensor, with the LED and photodiode at substantially the same temperature, without employing a procedure to independently change the temperature of the LED and a photodiode. Thus, in practice, it has been considered difficult to independently measure the change in optical properties of the LED and photodiode with temperature. If it was found that the photodiode output current dropped 10% due to a given change in temperature, this would imply that it would not be possible to determine to what extent that variation arose from changes in the optical properties of the LED, or optical properties of the photodiode, and so there would be no benefit to independently measuring the temperature of the LED and photodiode in use, as the effects of temperature on each component could not be separated.
The invention concerns the problem of manufacturing and calibrating optical absorption gas sensors having an LED and photodiode, to provide an output signal during operation of the resulting gas sensor which is accurate despite significant possible temperature differences between the LED and photodiode. Addressing this problem allows greater design freedom allowing more accurate, more energy efficient or cheaper sensors to be manufactured, and a further aspect of the invention concerns an improved configuration for an optical gas sensor waveguide.